The current widespread use of portable electronic devices such as mobile phones, PDAs, and portable computers, including a plurality of embedded systems powered by batteries, has directed research towards solutions that enable an even greater reduction in power consumption, in order to prolong the operational functioning of such devices before their batteries need to be recharged or replaced.
To achieve this goal, for a long time a technological process has been initiated for reducing the operating voltages (voltage scaling) of the components included in portable electronic devices, making it possible to develop systems with low power consumption.
However, due to certain operational specifications, it may not be possible to reduce the operating voltage of some of the electronic systems included in a portable electronic device, such as, for example, the memories of the EEPROM and FLASH types, which typically use voltages values higher than the voltage value that supplies the portable electronic device in which they are disposed in order to correctly carry out write and erase operations.
Integrated circuits called voltage multipliers have been made to resolve this issue. A voltage multiplier is a circuit that is able to generate voltages of a value higher than the supply voltage. The currently most used type of voltage multiplier is a charge pump. The charge pump voltage multipliers, or more simply charge pumps, are formed by a plurality of cascaded multiplication stages, each including a corresponding pumping capacitor. The operation of a charge pump is based on the accumulation and transfer of charge in the sequence of pumping capacitors, which are intercoupled to each other via corresponding switching elements, such as MOS transistors or diodes. In particular, each pumping capacitor has a free terminal, which is controlled by a signal that switches between a low voltage and a high voltage; the control signals of adjacent pumping capacitors are always in counter phase. In this way, when the control signal is the at low voltage value, the pumping capacitor is charged by the preceding pumping capacitor; when the control signal switches to the high voltage value, the stored charge is transferred to the following pumping capacitor.
Once a voltage multiplication operation is ended, the electric charge stored on the plates of the capacitors (which may reach very high levels) is discharged in order to avoid damaging the electronic components directly or indirectly supplied by the charge pump, as well as to avoid damaging the capacitors themselves and the switching elements of the charge pump.
Several solutions have been proposed for discharging the capacitors of charge pumps in a controlled manner. A simple proposed solution includes a single transistor controlled by a signal for enabling/disabling the charge pump. The assertion of the enabling/disabling signal causes the activation of the transistor, allowing the capacitors to discharge through a path towards a terminal at a lower voltage (typically, the supply voltage or the ground reference voltage of the system). Such a solution uses a transistors able to withstand high voltage differences (higher than the supply voltage of the system) between their terminals. Therefore, such transistors may require a dedicated production process (not always available) to be manufactured, thereby increasing the overall cost of the system.
Another known solution differs from the previous one since it provides for using a transistor that operates with reduced voltage differences (i.e. between the ground voltage and the supply voltage), and driving such transistor with a suitable level shifter circuit. This solution has the disadvantage of not fully discharging the charge pump because the transistor requires a minimum value for the overdrive voltage (i.e. the difference between the gate-source voltage and threshold voltage of the transistor) in order to remain active.
Another solution, disclosed in U.S. Pat. No. 5,537,072, which is incorporated by reference, proposes a switching circuit for a charge pump. The switching circuit has a first transistor for conducting a current and is controlled by a second, third, and fourth transistor. The second transistor protects the first transistor from an excessive voltage between the gate and drain terminals. The third transistor receives a signal for switching the switching circuit and serves as a cascode transistor to protect the fourth transistor from an excessive voltage between the gate and drain terminals. Consequently, the switching circuit may withstand high voltage values between the gate and drain terminals and may have an improved reliability. The switching circuit also has a shutdown circuit for facilitating the shutdown of the charge on the control element of the first transistor. The switching circuit also has a zener diode to prevent an excessive voltage from being applied between the gate and drain terminals of the first transistor.
Another known solution is described in U.S. Pat. No. 7,142,041, which is incorporated by reference and which describes a circuit and a method to shutdown a charge pump having any number of stages. The shutdown may be performed either until all the stages reach a null voltage or until all stages have the initial input voltage level. The shutdown is performed in a modular fashion, proceeding backwards from the output until reaching the input, so that the charge sharing among the capacitors is in such a way that no voltage exceeds its range of operation. An additional advantage of this solution is that the charge pump may be turned on before the shutdown sequence is completed, with all the internal and external nodes of the charge pump remaining within their normal range of operation.